As magnetic recording density has become higher, a spin-valve type giant magnetoresistive head has been used for a read head of an HDD (Hard Disk Drive) output signal of the read head has been increasing along with improvement on film structure. The spin-valve type giant magnetoresistive head has the film structure in which an antiferromagnetic layer, a ferromagnetic layer, a non-magnetic intermediate layer, and a soft magnetic free layer are deposited in that order. As magnetization of the ferromagnetic layer is pinned by an exchange coupling field occurring to an interface between the antiferromagnetic layer, and the ferromagnetic layer while magnetization of the soft magnetic free layer is reversed by an external field, relative orientations of the magnetization of the two magnetic layers are changed according to the external field, thereby causing electric resistance to undergo a change, so that a magnetic field is detected from the change in the electric resistance. In this case, current flows in a direction parallel to a film plane. This method for causing current to flow in the direction parallel to the film plane is generally called the CIP (Current-in-Plane) method.
In order to achieve a further output, research and development has lately been conducted on the CPP (Current Perpendicular to Plane)-GMR (Giant Magneto Resistive) head for causing current to flow in a direction perpendicular to the film plane, and a TMR (Tunneling Magneto Resistive) head. The TMR head features a high magnetoresistance ratio as it exhibits magnetoresistance by making use of a spin-dependent tunneling effect. Despite that the magnetoresistance ratio is large, a resistance area product (RA) is as large as several Ω/μm2. Accordingly, when a device becomes small in size, a head resistance increases, so that high-frequency characteristics are poor, thereby the TMR head has a disadvantage for fast transfer. Meanwhile, with the CPP type head, a proposal has been made on an artificial lattice type magnetoresistive device where a magnetic layer and a non-magnetic layer are deposited one after another. With the artificial lattice magnetoresistive device, because of a high probability of a sense current cutting across an interface between the magnetic layer and the non-magnetic layer, it is possible to obtain a large magnetoresistance ratio proportional to a probability of interface scattering. On the other hand, however, in the case where the CPP artificial lattice type magnetoresistive device is used for the head, it is necessary to decrease Mst (magnetization×total film thickness of a free layer) of a magnetic free layer in order to render the device highly sensitive to the external field. In the case of the artificial lattice magnetoresistive device, however, it is required that the magnetic layer and the non-magnetic layer should be alternately deposited many times in order to increase a resistance, which causes Mst to increase. In consequence, it becomes difficult to enhance sensitivity to the external field.
As another structure of a spin-valve structure, a proposal has been made on the CPP-GMR device for causing current to flow in the direction perpendicular to the film plane. With this structure, the total film thickness of the free layer is decreased as compared with the artificial lattice type device, so that higher sensitivity to the external field can be expected. Nevertheless, there exists a problem that a decrease in the number of the interfaces between the magnetic layer and the non-magnetic layer will result in a decrease in the magnetoresistance ratio. Further, the CPP-GMR device has a drawback in that RA of the device is low because the same in its entirety is formed of metal films, and unless the device is processed fairly small to thereby increase the head resistance thereof, it is not possible to obtain a sufficient output.
In this connection, in Japanese Patent Application No. 3293437, there has been proposed a magnetoresistive device with a non-magnetic film made up of a mixture of an insulator and an electrical conductor, inserted therein. Further, in connection with specific formation of a non-magnetic film, and method for manufacturing the same, in Japanese Patent Application No. 2003-298143, there has been proposed a method whereby an alloy made of not less than two kinds of elements is caused to undergo solid-solid phase segregation by use of a mechanism, such as spinodal decomposition, formation of a GP zone, and so forth, thereby preferentially causing one of the elements to undergo oxidation. Thus, with the spin-valve structure, as a result of the formation of a layer made up of a composite of the insulator and the electric conductor, the current flowing in the direction perpendicular to the film plane preferentially flows through an electrically conducting region of the non-magnetic film, so that it is possible to increase the device resistance and magnetoresistance ratio of the device.
The spin-valve type CPP-GMR head has a problem in that the magnetoresistance ratio thereof is still too small to achieve an areal density of the next generation, at 100 odd Gbits/in2, and the sensitivity of the magnetoresistive device is insufficient.